1
Q
The formation of glacier ice
A
- the first stage of formation of an ice mass is the accumulation of a permanent snowfield, either at high altitudes in mountainous areas or high latitude polar regions.
- There must be low numbers of positive degree days (when temperatures are above 0 °C) so that the snowfield can survive the summer melt and gradually increase each winter.
- The lower layers of this granular snow (density 0.1 g cm-3) become increasingly compressed to form névé or firn.
- Pressure-induced melting and refreezing of water filling gaps between individual ice crystals is the cause of this increasing density.
- As the snowfield increases in size, the process continues and the deeper layers, or firn, are transformed into glacier ice (density 0.9 g cm-3).
- The glacier is then deformed by further pressure and moves away from the centre and flows outward (in the case of an ice sheet/cap) or downward (in the case of a glacier) by extrusion flow.
2
Q
Névé (or firn):
A
Crystalline or granular snow, especially on the upper part of a glacier, where it has not yet been compressed into ice.
3
Q
Extrusion flow:
A
The theory that glacier ice flows faster at depth.
4
Q
Timescale for the process of the formation of ice
A
- The transformation from snowflake to firn can be very quick in more temperate areas (a few days) but much slower in polar areas (over a decade).
- The final stage from firn to glacial ice may take as little as 25 years but up to 150 years in polar areas such as Greenland.
- Overall rates of transformation from snow to ice can be, on average, as little as 100 years in some temperate areas, but can take up to 4000 years in Antarctica.
5
Q
Inputs and outputs of the glacier system
A
- The glacier system is driven by inputs of energy from the Sun (which evaporates water from the oceans to create air masses, which can produce snowfall).
- Mass enters the systems in the form of snowfall and rock debris.
- As this mass generally occupies an elevated position in the Earth’s gravitational field, this mass has potential energy, which is expended as the glacier flows downslope.
- The energy expended is used to warm or melt ice and then must be dissipated from the system in the form of heat and water.
- As this is going on, potential energy is turned into work, transferring ice and rock from the highlands towards lower levels and the oceans.
6
Q
Glacier mass balance
A
Mass balance is defined as the gains and losses of the ice store in the glacier system
7
Q
Accumulation and ablation
A
- Accumulation results from direct snowfall and other precipitation, ice, blown snow and avalanching from slopes above the glacier surface.
- The snow and ice are then transferred down valley by glacier movement until they reach lower areas where they are lost to the system either by melting, evaporation (sublimation) or the break away of ice blocks and icebergs (calving), processes collectively known as ablation.
- At the same time, there is an input and output of rock debris. Rock debris supplies come from weathering and erosion of slopes above the glacier; it is transported and eventually deposited as a further glacier output.
8
Q
equilibrium point of the glacier
A
- you get more accumulation than ablation in the upper part of the glacier, and more ablation than accumulation in the lower part of the glacier.
- The place where accumulation and ablation balance each other out is known as the glacier’s equilibrium point.
- Glacier systems are dynamic; the ratios of inputs to outputs vary continually over both short- and longer-term timescales.
- The adjustment of the glacier system to changes is reflected by variations in the mass balance.
9
Q
postive vs negative regimes
A
- If accumulation exceeds ablation, a usual situation in the winter period when there are very few positive degree days, the glacier increases in mass, i.e. a positive regime in the glacial budget.
- Conversely in summer, when there is more ablation than accumulation (rising temperatures) the glacier has a negative regime in the glacial budget.
- If the accumulation in winter is equalled by the ablation in summer, then the annual net balance is zero and the glacier is likely to be at a still-stand. Even within the time span of the annual budget, the regimes have some visual impact on the size of glacier mass.
- A positive regime or mass balance causes the glacier to grow and therefore advance at the snout.
- A negative regime or mass balance causes the glacier to thin/shrink/downwaste and therefore the position of the snout begins to retreat.
10
Q
Diagram of Annual mass balance of a typical glacier
A
11
Q
Longer term the situation of glaciers
A
- Currently it is estimated that nearly 75 per cent of the world’s ice masses are experiencing rising trends’ in their net negative balances, almost certainly as a result of short-term climate change (the average global increase in temperatures is 0.6 °C in the last decade) and are thus thinning, melting and retreating.
- In the past much of the evidence of longer-term fluctuations in the mass balance of glaciers was carried out using old photographs and maps, augmented by field surveys of the lateral and end margins (snout)
12
Q
The Greenland Ice Sheet
A
- The Greenland Ice Sheet is one of the world’s two remaining ice sheets. It currently covers an area of 1.7 million km2.
- As it contains more than 2.5 million km3 of stored ice, it has a huge potential impact on other earth systems including the atmosphere and oceans.
- In the centre of the ice sheet the ice is over 3 km thick, so its weight isostatically depresses the earth’s crust by about 1 km in depth.
- In response to recent climate warming (very marked in the Arctic area) a number of changes are occurring to the mass balance of the ice store (figures indicate change per year in water km3):
- Accumulation from snowfall in the central area: +520 km
- Ablation by melting and edges: -290
- Ablation by calving icebergs: -200
- Ablation by sublimation: -60
- total ablation: -550
- Mass balance: -30
13
Q
The Greenland Ice Sheet - feedback cycles
A
- Cumulative study of research findings suggests that overall the Greenland Ice Sheet has a negative mass balance with an accelerating rate of increase.
- There is huge uncertainty as to how the mass balance of the huge ice store could change in the future largely because of feedback loops.
- Positive feedback loops amplify the speed of any processes whereas negative feedback loops diminish their impact
- An example of positive feedback could occur if as a result of anthropogenic processes the Greenland Ice Sheet melted catastrophically, for example, snow melt on the edge of the ice sheet would lead to widespread occurrence of bare ground.
- This reduces the albedo of the surface (of snow cover) so accelerating warming of the land.
- Moreover, increased melting could lead to the release of huge quantities of methane, a naturally occurring greenhouse gas stored in tundra zones, accelerating ablation.
- If the Greenland Ice Sheet was to melt completely the height of the land would be so low that surface temperatures could be much warmer although isostatic recovery would counteract this and the amount of sea level rise.
- So the negative mass balance accelerating would have a huge impact on sea levels with a possible rise of 7 m globally.
- However, the rapid melting of the ice sheet would upset the thermohaline circulation cutting off warm water currents from the Gulf Stream from reaching countries such as the UK.
- This negative feedback occurs as the absence of warm currents diminishes the impacts of climate warming.
14
Q
Similar research is also taking place in
A
- other remote regions, such as Greenland, Svalbard and northern Canada (Baffin and Ellesmere Islands).
- The USGS Benchmark Glacier Research project measures changes in the mass balance for four benchmark glaciers: Gulkana and Wolverine in Alaska, South Cascade in Washington State and, most recently, Sperry in Montana (since 2005).
- These four research sites were unified into a single project with common strategies for field work analysis and research methodology to enable comparison between the glaciers and to measure each glacier’s exposure to climate change.
- This project again provides long-term records of mass balance trends at a continental scale.
15
Q
Benchmark glacier:
A
A designated glacier in which the accumulation and ablation are measured annually using standardised techniques to monitor the impacts of climate change. USGS, for example, studies five such glaciers currently with more chosen for the future to reflect a variety of locational scenarios.
